Pressure sensors are sensors used in various fields. Of these, the demands for a small-sized pressure sensor formed by using MEMS (Micro Electro Mechanical Systems) manufacturing technologies or semiconductor microfabrication technologies have been increasing rapidly. For example, for industrial use, such sensors have been applied to pressure control and monitoring of various types of plant equipment. For consumer use, they have been used for gas meters, flowmeters, sphygmomanometers, and others. For automobile, they have been used for engine or brake control, tire pressure monitoring, and others.
In view of means for detecting pressure, the small-sized pressure sensors can be classified into those of a piezoresistance type that use a piezoresistive element embedded in a diaphragm to detect a deflection of the diaphragm caused by pressure, those of a capacitance type that detect a distance between two electrodes varying by pressure as a change in capacitance, those of a resonator type that detect a change in resonance frequency of a resonator caused by a change in pressure, and others.
On the other hand, as pressure sensors directed to the purposes other than those of the small-sized pressure sensors mentioned above, pressure-sensitive sensors have also been increasingly developed. Such a sensor is mounted on the head or leg portion of a pet robot, for example, for use in detecting a contact with a human. The sensor is sometimes used at the input unit of an input device for adjusting the rotation speed of a motor or the sound volume of a speaker. The pressure-sensitive sensor detects a pressure by means of a pressure-sensitive conductive film, for example. The value of the resistance of the pressure-sensitive conductive film varying in accordance with a deformation by pressure is detected.
Note that one example of the pressure-sensitive sensor using a pressure-sensitive conductive film is disclosed in Japanese Patent Laid-Open Publication No. 9-17276 (Patent Document 1). Also, one example of the pressure-sensitive conductive film and the manufacturing method thereof are disclosed in Japanese Patent Laid-Open Publication No. 2000-299016 (Patent Document 2).
Particularly in a small-sized pressure sensor for automobiles, demands for size reduction, performance enhancement, and cost reduction are generally strong. Especially, the reduction in size and weight is particularly important for a pressure detecting unit used in a tire pressure monitoring system (TPMS), due to its characteristics of being installed in a tire for a long time. Also, since power is often fed through a button battery, low power consumption is also desired. In the initial stage of the practical use of the sensors, a method in which components including a piezoresistive monolithic pressure sensor, a semiconductor device for signal processing, a wireless semiconductor device, a button battery and others are mounted on a small-sized printed board was used. However, due to problems such as high cost and large power consumption leading to a short battery life, such a method has not yet been widespread.
For the achievement of size reduction, low power consumption and low cost, it is effective to accommodate, in addition to the monolithic pressure sensor, peripheral semiconductor devices required for processing and transmitting a signal from the sensor in a single package, and further, in a single chip. This has been actively studied.
The single-packaging has been achieved by mounting a piezoresistive pressure sensor or the like and a semiconductor device having a function to amplify a signal from the sensor and other functions in a single package by using a mounting technology. An example of such single-packaging is disclosed in J. Dancaster et al., “TWO-CHIP PRESSURE SENSOR AND SIGNAL CONDITIONING”, TRANSDUCERS '03 (The 12th International Conference on Solid State Sensors, Actuators and Microsystems, Boston, Jun. 9-12 2003) proceedings, pp. 1669-1702 (Non-patent Document 1). Since this single-packaging allows the pressure sensor and the semiconductor device to be manufactured separately, each device structure and manufacturing process does not have to be changed, which is advantageous. However, since the pressure sensor and a part of the semiconductor devices are simply packaged as one, it is difficult to achieve the cost reduction. Although a piezoresistive pressure sensor from which a large output signal can be obtained is often used, such a piezoresistive pressure sensor requires large power consumption. In the non-patent document 1, effects of low power consumption achieved through the contrivance on the circuitry are described. However, effects of low power consumption achieved by the single-packaging itself is considered to be small.
On the other hand, the single-chip has been achieved by a manufacturing method in which two substrates are bonded together. A recent example of such a method is described in detail in Abhijeet V. Chavan et al., “A Monolithic Fully-Integrated Vacuum-sealed CMOS Pressure SENSOR”, IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 49, NO. 1, JANUARY 2002, pp. 164-169 (Non-patent Document 2). In this method, a substrate on which at least a part of a pressure sensor is formed and a substrate on which a semiconductor device or the like is formed are bonded together by means of anodic bonding or the like to form one substrate. Another exemplary method is a manufacturing method in which, after a semiconductor device is formed on the perimeter of a semiconductor chip, the rear surface at the center portion of the chip is processed into a thin film by the etching using potassium hydroxide to form a diaphragm, and a glass substrate is laminated on the rear surface side to form a vacuum cavity portion. In either case, for saving the manufacturing effort and reducing the cost, a method of bonding during a substrate stage before dicing is generally taken. Thereafter, by dicing the bonded substrates, a pressure sensor and a semiconductor device are embedded in one chip. What is used in the Non-patent Document 2 is a capacitive pressure sensor. Since the single-chip made by this method can shorten the wiring length between the pressure sensor and the detecting circuit, a capacitive sensor required to detect a minute change in capacitance can be applied. However, similar to the above-described single-packaging, the single-chip made by this method has a limitation on the cost reduction.
A method having a higher possibility for suppressing manufacturing cost and achieving further size reduction and low power consumption than that of the above-described single-packaging or single-chip made by bonding substrates is a method of forming both of a pressure sensor and a semiconductor device together on a semiconductor substrate, with a contrivance on device structure and manufacturing process. Depending on the combination of the type of the pressure sensor and the semiconductor device for realizing a single-chip, some contrivance will be required for the device structure and the manufacturing process. Methods of forming a capacitive pressure sensor while manufacturing a semiconductor device are disclosed in U.S. Pat. No. 6,472,243 (Patent Document 3), Klaus Kasten et al., “CMOS-compatible capacitive high temperature pressure sensors”, Sensors and Actuators 85 (2000), pp. 147-152 (Non-patent Document 3), and Klaus Kasten et al., “High temperature pressure sensor with monolithically integrated CMOS readout circuit based on SIMOX technology”, The 11th International Conference on Solid-State Sensors and Actuators (Munich, Germany, Jun. 10-14, 2001) proceedings, pp. 510-513 (Non-patent Document 4). With these manufacturing methods, some products in which CMOS (Complementary Metal Oxide Semiconductors) including analog/digital combined circuits such as a temperature sensor for temperature compensation, an analog-digital converter circuit, a logic circuit, a clock and a power supply control circuit are combined in addition to the pressure sensor in one chip have already been in practical use. In some cases, non-volatile memory such as EEPROM (Electrically Erasable and Programmable Read Only Memory) for storing calibration data or the like may be also embedded.
In the conventional technologies described in Patent Document 3 and U.S. Pat. No. 5,596,219 (Patent Document 4), Non-patent Document 3 and 4, and T. Bever et al., “Solutions for The Pressure Monitoring Systems”, 7th International Conference on Advanced Microsystems for Automotive Applications 2003 (Berlin, Germany, May 22-23, 2003) proceedings, pp. 261-269 (Non-patent Document 5), a polycrystalline silicon layer is used as a diaphragm serving as an upper electrode of a capacitor for obtaining a capacitance in accordance with the pressure. As a lower electrode, another polycrystalline silicon layer different from a diaphragm formed on a diffusion layer in the substrate or a field oxide film is used.
The reason why the conventional technologies select the above structure is to form a capacitor having a shape close to that of a parallel plate capacitor. To ensure durability of the diaphragm and approximate a capacitor with its capacitance varying linearly with respect to pressure, a parallel plate capacitor is considered to be most advantageous.
In general, in the course of manufacturing a semiconductor device, the topography gets less planar. In some cases, a planarization process may be inserted to achieve more planar topography. However, in a general method of manufacturing a semiconductor device particularly with its critical dimension larger than 0.5 μm, even if local nonplanar topography are mitigated, nonplanar topography in a large area such as that required by a capacitor are not resolved. That is, it is extremely difficult to form a parallel plate capacitor on an upper portion where a semiconductor element such as a transistor has been once formed. When a pressure sensor is embedded together with the semiconductor device produced by such a manufacturing method, there is no other way but to secure a flat area on the diffusion layer or the field oxide film and then form a capacitor for detecting pressure thereon.
One reason for using a polycrystalline silicon layer as the electrode material is that the electrode can be formed simultaneously during the process of forming a polycrystalline silicon layer serving as a gate electrode or a resistive layer of a transistor. Also, since it is resistant to hydrofluoric acid, polycrystalline silicon can be used as a mask when a silicon oxide layer between electrodes of the capacitor is etched, thereby advantageously making it easy to form the capacitor. Furthermore, since polycrystalline silicon has been studied for a long time as a material for movable parts of MEMS, which film to be used for forming an excellent diaphragm has been greatly elucidated. Also at this point, polycrystalline silicon can be considered as an easy-to-use material.